This invention is related generally to ultrasonic systems for measuring the flow of liquid through a pipe, and in particular to a system whose receiving stage includes an automatic gain-controlled amplifier responsive only to the first pulsatory swing in the received signal burst.
The use of ultrasonic techniques to determine the flow rate or other characteristics of a liquid flowing through a pipe is well known. Among the patents disclosing systems of this type are U.S. Pat. Nos. 4,103,551; 4,004,461 and 3,906,791. Typically, in an ultrasonic system adapted to measure flow rate, a pulse of ultrasonic energy is alternately emitted by a pair of transducers, one transducer being placed upstream on the pipe at one end of a tilted diameter or diagonal, the other being placed downstream at the other end of the diagonal. The direction of tilt lies in the direction of flow.
A pulse of ultrasonic energy emitted by one transducer and propagated through the fluid being metered is detected by the other transducer. The time delay difference (that is, the upstream time minus the downstream time) between the generation of the emitted pulses and their reception is a function of flow velocity, and therefore may be converted into a flow rate reading.
When a strong pulse of ultrasonic energy is transmitted through a pipe wall and then through the fluid carried by the pipe before being picked up by a transducer, the received signal burst has a relatively low amplitude. The received signal must, therefore, be amplified to a level sufficient to operate a comparator to produce pulses whose time position reflects the transit time through the fluid. Conventionally, the amplifier for this purpose is either one having manual gain control means or an automatic gain control circuit.
The disadvantage of a manually-operated gain control is that while it may be set to afford the proper degree of gain, this setting does not take into account subsequent changes in received signal strength which normally occur over a period of time in an ultrasonic measuring system installation.
To avoid the need to make manual adjustment in amplifier gain from time to time, one may use a standard automatic gain control (AGC) arrangement to maintain the proper level of gain. An AGC amplifier is one whose gain is automatically regulated so that its output signal remains at a desired level despite variations in the strength of the signal.
It is, therefore, the usual practice in an ultrasonic measuring system to make use of an AGC amplifier to provide a degree of tolerance with respect to the many variations normally encountered in the content of the received ultrasonic signal burst. An AGC amplifier is normally responsive to the strongest pulsatory swing in the received signal burst to establish the operative AGC level.
In the received ultrasonic signal burst, the first swing is generally smaller than the second and is often of lesser amplitude than the third, fourth and fifth signal swing in the signal burst. But whichever subsequent swing has the greatest amplitude, this acts to govern the operative AGC level. Because of variations encountered in the overall strength of the received signal burst, in the number of swings included in the burst and the shape of the burst envelope, the response of the AGC circuit will inevitably be jumpy. This condition gives rise to erratic operation of the gain-controlled amplifier and may produce a considerable measurement error.
Another factor which comes into play in some ultrasonic flow rate measuring systems occurs when upstream and downstream transducer roles are interchanged, causing the frequency of the received signal burst to undergo a step which is experienced as a difference in the period of time between the first and second swing in the signal burst. The time error resulting from this effect can best be minimized if the ACG amplifier is responsive only to the first signal swing.